It is found that the sensitivity of photoresponse of SnO2 nanowires can be enhanced by metallic particles decoration. The underlying mechanism is attributed to the formation of the Schottky junction on nanowires surface in the vicinity of metallic nanoparticles. The increment in the barrier height and width of space charge region due to the existence of Schottky junction increases the surface electric field and enhances the spatial separation effect, which then prolongs the lifetime of photoinduced electron and consequently increases the photoresponse gain. The result shown here provides an alternative route for enhancing the photoresponse of semiconductor nanostructures, which should be useful for creating highly sensitive photodetectors.
© 2008 Optical Society of America
Recently, metal-oxide semiconductors based on one-dimensional (1D) nanostructures have been extensively investigated for their novel properties and attractive applications . Among them, SnO2 is a typical n-type semiconductor with a wide direct band gap of 3.6 eV, which can particularly be applied as a good visible-blind ultraviolet (UV) detector . Due to its chemical stability and optoelectronic characteristics, SnO2 nanowires (NWs) are also broadly used in electrodes, solar cells, and many other photoelectronic devices [3–6]. In addition, because of a large surface-to-volume ratio of 1D nanostructures, significant progress has been reported for the application in gas sensors such as the detection of CO, NO, and H2 [7–9].
Currently, in order to improve the sensitivity of gas sensors, it has been proposed to incorporate catalytically active metals, such as Pd and Ag, on semiconductor NWs [10,11]. The metal nanoparticles on the surface will results in the formation of a localized Schottky junction, which creates a charge depletion region in the NWs in the vicinity of the metal particles. The metal-particles could then become an intermediate in chemical combustion reactions which reduce the current through the NWs and then enhance the sensitivity. In this paper, we report that the photoresponse of SnO2 NWs with Au-nanoparticles decoration can be greatly enhanced by up to 110%. Our finding can be well interpreted based on the formation of the Schottky junction on the decorated surface in which the increased surface electric field will enhance the spatial separation of photoexcited electrons and holes, thereby the photocurrent gain is amplified.
2. Experimental setup
The growth of the studied SnO2 NWs was based on the method named vapor-liquid-solid (VLS). In the synthesizing process, the Au layer with 10 nm in thickness is first deposited on M-plane (100) sapphire (Al2O3) to serve as the catalyst. Then Sn powder is placed on the ceramic boat and put into a furnace with Argon flow as gas carriers at a flow rate of 200 sccm. The temperature is rapidly increased from room temperature up to 1000 °C by a rate of about 100°C/min. After the vaporized Sn is mixed with oxygen in air and turns into SnO2, SnO2 is then blown onto the Au layer. When SnO2 dissolves into Au, the supersaturation of the alloy SnO2 -Au droplet results in the nucleation, and SnO2 NWs are fabricated. The as-grown SnO2 NWs have diameters of 100~300 nm and length above 40 µm (not show here). After the fabrication, the electric contacts of SnO2 NWs are made by a well-known method named shadow-mask technique. A detailed description of the technique can be found elsewhere, and we will not repeat here. Sapphire is chosen to be the nonconducting substrate, and metallic Ni (15 nm)/Au (150 nm) electrodes were vapor deposited. A post treatment of annealing at 500 °C for 60 s after the contact is deposited was necessary to achieve the ohmic contact.
A scanning electron microscopy (SEM) image of the sample is shown in the inset of Fig. 1. The distance of the channel between two terminals is 10 µm, and the diameters of the three NWs are respectively 137, 295, and 479 nm. The dark current versus bias (I–V) measurement is routinely checked to ensure ohmic contact between nanowires and metal electrodes as shown in Fig. 1. The x-ray diffraction (XRD) shown in the inset of Fig. 1 is employed to identify the structure of synthesized products. Most of the peaks can be perfectly indexed according to the tetragonal rutile structure of SnO2.
For the photocurrent (PC) measurement, in order to generate the free electron-hole pairs, a He-Cd laser working at 325 nm was used as the excitation light source, whose energy is larger than the band gap energy of SnO2 (~345 nm). A measurement system (Keithley 236) was utilized to supply the dc voltage (0.1 V) and to record the photocurrent. Au-nanoparticles were sputtered on the NWs with the JFC Coater in low-pressure ambient environment (below 5 Pa). The JFC Coater was performed by applying a high voltage to create gas ions which will bombard Au target, and generate Au nanoparticles from the target to be ejected with enough energy to travel to sample.
3. Results and discussion
As shown in Fig. 2(a), the diameter of the Au-nanoparticles is about 10 nm, and the random distribution of the Au-nanoparticles does not cover the whole surface of the NWs. Thus the decorated Au-nanoparticles will not screen the incident light, and not be able to produce a new conductive path on the surface and the substrate. As shown in Fig. 1, the I–V curves of the NWs show a well-behaved ohmic characteristic. Figure 2(b) shows the results of photoresponse under different excitation intensity performed on SnO2 NWs in ambient air. It is clear that the PC increases with increasing light intensity from 2.5 W/m2 to 774 W/m2. Quite interestingly, when the NWs are decorated with Au-nanoparticles, the measured PC can be enhanced by up to about 110%. It will be intriguing to know the underlying mechanism that causes the PC enhancement after Au-nanoparticles decoration.
The photoresponse can be quantitatively described by current gain (Γ), a factor to determine the efficiency of electron transport and carrier collection during illuminating process. The current gain (Γ) indicates the number of electrons induced by photon, which can be obtained by 
where Δi is the current difference between photocurrent and dark current, q the electron charge, hν the photon energy of incident light of 3.81 eV. P is the power of incident photon that has been absorbed by the NWs, which means P=I×l×d. I is the excitation light intensity on the sample, l and d are respectively the length and the width of the NWs. η is the quantum efficiency which is set to be 1 for simplicity. For the SnO2 NWs studies here, the measured PC has an extremely high PC gain with a value up to about 900.
The high current gain in SnO2 has been attributed to the presence of oxygen vacancies. Due to the existence of oxygen vacancies on the surface of SnO2 NWs, free electrons are accumulated on the surface . Consequently, the existence of upward band-bending forms a low-conductivity depletion region near the surface referred to space charge regions (SCRs). After the electron-hole pairs are photogenerated, the photoinduced holes can migrate to the surface by the built-in electric field. As a result, a conductive volume increment is produced. And the spatial separation of electrons and holes also reduces the electron-hole recombination rate, and therefore, the electron lifetime increases and the photoresponse is enhanced. According to the simulation of Garrido et al , the PC induced by the modulation of surface SCRs causes Γ following an inverse power law with excitation intensity, i.e., Γ∝ I-K, and the exponent K is between 0.5 and 0.9. As shown in Fig. 3, the gain logarithmic plot in the intensity ranging from 2.5 W/m2 to 774 W/m2 shows a well defined power law with K~0.601± 0.009 which is in good agreement with the theoretical prediction.
Let us now try to understand the origin of the PC enhancement after the Au decoration. It is known that depositing the metallic particles on the surface of a semiconductor, such as SnO2 [10,11], results in a localized Schottky barrier in the vicinity of the metallic particles. The formation of the Schottky barrier on the surface will enhance the surface electric field and increase the width and height of space charge region as shown in Fig. 4.
This is due to the fact that the work function of SnO2 NWs has a value of 4.7 eV, which is smaller than that of Au cluster with 5.1 eV. The increase in the barrier height of SCRs on the surface will enhance the spatial separation of photoexcited electrons and holes, and the electron lifetime is enhanced. In turn, the measured PC is enlarged. In order to provide a further evidence to support the above proposed mechanism, we compare the dependence of the photoresponse on excitation intensity for NWs without and with Au-nanoparticles decoration. As shown in Fig. 3, the exponent K of inverse power law changes from 0.601± 0.009 to 0.647±0.010 after the decoration of SnO2 NWs.
This behavior can be understood according to the following equation. SCRs inside a semiconductor produce a variation of the conductive volume when carriers are photogenerated, and the Δi can be expressed as 
ε is the permittivity, and ΔΨo is the barrier height. N d is the dopping level, V T=kT/q and A* is Richardson constant=1.2×106 A/m2K2. Figure 5 shows the simulated result according to Eq. (2), which clearly indicates that both of the gain Γ and the exponent K increase with increasing barrier height. This result is again consistent with the proposed Schottky junction model as described above .
However, Fig. 1(b) shows that the current of Au-decorated SnO2 NWs before illumination is higher than that of pristine SnO2 NWs. The dark current does not decrease by the expansion of SCRs after the Au-decoration. This behavior can be understood in terms of the fact that a large number of electrons in Au-nanoparticles have been transferred into the conduction channel of SnO2 NWs . In addition, because of the enhanced band-bending, the conduction carriers are more concentrated on the center of the wires, which reduces the surface scattering and therefore the dark current conductivity is enhanced.
In summary, metallic decoration has been utilized to improve the sensitivity of photoresponse in SnO2 NWs. The underlying mechanism is attributed to the formation of the localized Schottky barrier in the vicinity of metallic nanoparticles, which enhances the surface electric field and increases the spatial separation of photoexcited electrons and holes. It thus prolongs the photoinduced electron lifetime, and increases the photocurrent gain. Besides, this enhancement also takes the advantage of the inherent nature of a large surface to volume ratio of 1D structure. Our result demonstrates that metal-particles decoration provides an alternative way for enhancing the photocurrent of semiconductor NWs, which should be very useful for creating highly sensitive photodetectors.
This work was supported by the National Science Council and Ministry of Education of Republic of China.
References and links
1. S. Dmitriev, Y. Lilach, B. Button, M. Moskovits, and A. Kolmakov, “Nanoengineered chemiresistors: the interplay between electron transport and chemisorption properties of morphologically encoded SnO2 nanowires,” Nanotechnology , 18, 055707–055712 (2007). [CrossRef]
2. Z. Liu, D. Zhang, S. Han, C. Li, T. Tang, W. Jin, X. Liu, B. Lei, and C. Zhou, “Laser Ablation Synthesis and Electron Transport Studies of Tin Oxide Nanowires,” Adv. Mater. (Weinheim, Ger.) 15, 1754–1757 (2003). [CrossRef]
3. Y. K. Liu, C. L. Zheng, W. Z. Wang, C. R. Yin, and G. H. Wang, “Synthesis and Characteristics of Rutile SnO2 Nanorods,” Adv. Mater. (Weinheim, Ger.) 13, 1883–1887 (2001). [CrossRef]
4. B. Wang, Y. H. Yang, C. X. Wang, N. S. Xu, and G. W. Yang, “Field emission and photoluminescence of SnO2 nanograss,” J. Appl. Phys. 98, 124303-1–12430-4 (2005). [CrossRef]
5. B. Wang, Y. H. Yang, C. X. Wang, and G. W. Yang, “Nanostructures and self-catalyzed growth of SnO2,” J. Appl. Phys. 98, 073520-1–073520-5 (2005).
6. S. Luo, P. K. Chu, W. Liu, M. Zhang, and C. Lin, “Origin of low-temperature photoluminescence from SnO2 nanowires fabricated by thermal evaporation and annealed in different ambients,” Appl. Phys. Lett. 88, 183112-1–183112-3 (2006). [CrossRef]
7. L. L. Fields, J. P. Zheng, Y. Cheng, and P. Xiong, “Room-temperature low-power hydrogen sensor based on a single tin dioxide nanobelt,” Appl. Phys. Lett. 88, 263102-1–263102-3 (2006). [CrossRef]
8. A. Yang, X. Tao, R. Wang, S. Lee, and C. Surya, “Room temperature gas sensing properties of SnO2/multiwall-carbon-nanotube composite nanofibers,” Appl. Phys. Lett. 91, 133110-1–133110-3 (2007).
9. S. Choudhury, C. A. Betty, K. G. Girija, and S. K. Kulshreshtha, “Room temperature gas sensitivity of ultrathin SnO2 films prepared from Langmuir-Blodgett film precursors,” Appl. Phys. Lett. 89, 071914-1–071914-3 (2006). [CrossRef]
10. A. Kolmakov, D. O. Klenov, Y. Lilach, S. Stemmer, and M. Moskovits, “Enhanced Gas Sensing by Individual SnO2 Nanowires and Nanobelts Functionalized with Pd Catalyst Particles,” Nano. Lett. 5, 667–673 (2005). [CrossRef] [PubMed]
11. X. H. Chen and M. Moskovits, “Observing Catalysis through the Agency of the Participating Electrons: Surface -Chemistry-Induced Current Changes in a Tin Oxide Nanowire Decorated with Silver,” Nano. Lett. 7, 807–812 (2007). [CrossRef] [PubMed]
12. C. Y. Chang, G. C. Chi, W. M. Wang, L. C. Chen, K. H. Chen, F. Ren, and S. J. Pearton, “Electrical Transport Properties of Single GaN and InN Nanowires,” J. Electro. Mater. 35, 738–743 (2006). [CrossRef]
13. R. S. Chen, H. Y. Chen, C. Y. Lu, K. H. Chen, C. P. Chen, L. C. Chen, and Y. J. Yang, “Ultrahigh photocurrent gain in m-axial GaN nanowires,” Appl. Phys. Lett. 91, 223106-1–223106-3 (2007).
14. X. T. Zhou, F. Heigl, M. W. Murphy, T. K. Sham, T. Regier, I. Coulthard, and R. I. R. Blyth, “Time-resolved x-ray excited optical luminescence from SnO2 nanoribbons: Direct evidence for the origin of the blue luminescence and the role of surface states,” Appl. Phys. Lett. 89, 213109-1–213109-3 (2006). [CrossRef]
15. J. A. Garrido, E. Monroy, I. Izpura, and E. Muñoz, “Photoconductive gain modelling of GaN photodetectors,” Semicond. Sci. Technol. 13, 563–568 (1998). [CrossRef]